Multi-stage compressor unit for refrigeration system

ABSTRACT

A multi-stage compressor unit for a refrigeration system configured to circulate a refrigerant comprises a first compressor sub-unit having a first stage and a second stage, and a second compressor sub-unit in parallel with the first compressor sub-unit and having a first stage. The first and second stages of the first compressor sub-unit each have a suction port and a discharge port. The first compressor sub-unit is configured to receive and compress a first portion of the refrigerant from an evaporator. The first stage of the second compressor sub-unit is configured to compress a second portion of the refrigerant.

BACKGROUND OF THE INVENTION

The present invention relates generally to compressors used in refrigeration systems. More particularly, the present invention relates to a multi-stage compressor unit for a refrigeration system that includes at least one two-stage compressor sub-unit.

A typical refrigeration system includes an evaporator, a compressor, a condenser, and a throttle valve. A refrigerant, such as a hydrofluorocarbon (HFC), typically enters the evaporator as a two-phase liquid-vapor mixture. Within the evaporator, the liquid portion of the refrigerant changes phase from liquid to vapor as a result of heat transfer into the refrigerant. The refrigerant is then compressed within the compressor, thereby increasing the pressure of the refrigerant. Next, the refrigerant passes through the condenser, where it changes phase from a vapor to a liquid as it cools within the condenser. Finally, the refrigerant expands as it flows through the throttle valve, which results in a decrease in pressure and a change in phase from a liquid to a two-phase liquid-vapor mixture.

While natural refrigerants such as carbon dioxide have recently been proposed as alternatives to the presently used HFCs, the high side pressure of carbon dioxide typically ends up in the supercritical region where there is no transition from vapor to liquid as the high pressure refrigerant is cooled. For a typical single stage vapor compression cycle, this leads to poor efficiency due to the loss of the subcritical constant temperature condensation process and to the relatively high residual enthalpy of supercritical carbon dioxide at normal high side temperatures.

Thus, there exists a need for a compressor unit for a refrigeration system that is capable of utilizing any refrigerant, including a transcritical refrigerant, while helping to maintain a high level of system efficiency.

BRIEF SUMMARY OF THE INVENTION

The present invention is a multi-stage compressor unit for a refrigeration system configured to circulate a refrigerant. The multi-stage compressor unit comprises a first compressor sub-unit having a first stage and a second stage, and a second compressor sub-unit in parallel with the first compressor sub-unit and having a first stage. The first and second stages of the first compressor sub-unit each have a suction port and a discharge port. The first compressor sub-unit is configured to receive and compress a first portion of the refrigerant from an evaporator. The first stage of the second compressor sub-unit has a suction port and a discharge port. The second compressor sub-unit is configured to compress a second portion of the refrigerant

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1A illustrates a schematic diagram of a first alternative embodiment of a multi-stage compressor unit connected to a refrigeration system.

FIG. 1B illustrates a graph relating enthalpy to pressure for the refrigeration system of FIG. 1A.

FIG. 2A illustrates a schematic diagram of a second alternative embodiment of a multi-stage compressor unit connected to a refrigeration system.

FIG. 2B illustrates a graph relating enthalpy to pressure for the refrigeration system of FIG. 2A.

FIG. 3A illustrates a schematic diagram of a third alternative embodiment of a multi-stage compressor unit connected to a refrigeration system.

FIG. 3B illustrates a graph relating enthalpy to pressure for the refrigeration system of FIG. 3A.

FIG. 4A illustrates a schematic diagram of a fourth alternative embodiment of a multi-stage compressor unit connected to a refrigeration system.

FIG. 4B illustrates a graph relating enthalpy to pressure for the refrigeration system of FIG. 4A.

FIG. 5A illustrates a schematic diagram of a fifth alternative embodiment of a multi-stage compressor unit connected to a refrigeration system.

FIG. 5B illustrates a graph relating enthalpy to pressure for the refrigeration system of FIG. 5A.

FIG. 6A illustrates a schematic diagram of a sixth alternative embodiment of a multi-stage compressor unit connected to a refrigeration system.

FIG. 6B illustrates a graph relating enthalpy to pressure for the refrigeration system of FIG. 6A.

DETAILED DESCRIPTION

FIG. 1A illustrates a schematic diagram of multi-stage compressor unit 10A connected to refrigeration system 20A, which includes heat rejecting heat exchanger 24, first economizer circuit 25A, main expansion valve 26, evaporator 27, and sensor 31. First economizer circuit 25A includes first economizer heat exchanger 28A, expansion valve 30A, and sensor 31A. Although first economizer heat exchanger 28A is depicted as a parallel flow tube-in-tube heat exchanger, multi-stage compressor unit 10A is useful in refrigeration systems utilizing other types of economizer heat exchangers including, but not limited to, counter flow tube-in-tube heat exchangers, shell-in-tube heat exchangers, flash tanks, and brazed plate heat exchangers.

Multi-stage compressor unit 10A includes two-stage compressor sub-unit 32 and single-stage compressor sub-unit 34. As shown in FIG. 1, two-stage compressor sub-unit 32 is a reciprocating compressor and includes cylinders 36A and 36B connected in series. Similarly, single-stage compressor sub-unit 34 is also a reciprocating compressor and includes cylinder 36C. Although two-stage compressor sub-unit 32 and single-stage compressor sub-unit 34 are shown as reciprocating compressors, other types of compressors (in various combinations) may be used including, but not limited to, scroll, screw, rotary vane, standing vane, variable speed, hermetically sealed, and open drive compressors. However, for purposes of example, embodiments of the present invention will be described as including reciprocating-type compressor units having multiple stages represented by compression cylinders.

In refrigeration system 20A, two distinct refrigerant paths are formed by connection of the various elements in the system. A main refrigerant path is created by a loop defined by the points 1, 2, 3, 4, and 5. A first economized refrigerant path is created by a loop defined by the points 4A, 5A, 6A, and 7A. It should be understood that the paths are all closed paths that allow for continuous flow of refrigerant through refrigeration system 20A.

In reference to the main refrigerant path, after refrigerant exits two-stage compressor sub-unit 32 at high pressure and enthalpy through discharge port 39 (point 3), the refrigerant loses heat in heat rejecting heat exchanger 24, exiting heat rejecting heat exchanger 24 at low enthalpy and high pressure (point 4A). The refrigerant then splits into two flow paths 40A and 42A prior to entering first economizer heat exchanger 28A. The main path continues along path 40A through first economizer heat exchanger 28A (point 4). As the refrigerant in path 40A flows through first economizer heat exchanger 28A, it is cooled by the refrigerant in path 42A of the first economized path.

Refrigerant from path 40A is then throttled in main expansion valve 26. Main expansion valve 26, along with economizer expansion valve 30A, is preferably a thermal expansion valve (TXV) or an electronic expansion valve (EXV). After going through an expansion process within main expansion valve 26 (point 5), the refrigerant is a two-phase liquid-vapor mixture and is directed toward evaporator 27. After evaporation of the remainder of the liquid (point 1), the refrigerant enters two-stage compressor sub-unit 32 through suction port 37. The refrigerant is compressed within cylinder 36A, which is the first stage of two-stage compressor sub-unit 32, and is then directed out discharge port 50 (point 2). After the second stage of compression, the refrigerant is discharged through discharge port 39 (point 3).

In reference to the first economized path, after refrigerant exits heat rejecting heat exchanger 24 at low enthalpy and high pressure (point 4A) and splits into two flow paths 40A and 42A, the first economized path continues along path 42A. In path 42A, the refrigerant is throttled to a lower pressure by economizer expansion valve 30A (point 5A) prior to flowing through first economizer heat exchanger 28A. The refrigerant from path 42A that flowed through first economizer heat exchanger 28A (point 6A) is then directed along economizer return path 46A and injected into suction port 52 of single-stage compressor sub-unit 34 for compression in single-stage compressor sub-unit 34. After compression within single-stage compressor sub-unit 34, refrigerant is discharged through discharge port 54 (point 7A) where it merges with the refrigerant discharged from two-stage compressor sub-unit 32.

Refrigeration system 20A also includes sensor 31 disposed between evaporator 27 and multi-stage compressor unit 10A along the main refrigerant path. In general, sensor 31 acts with expansion valve 26 to sense the temperature of the refrigerant leaving evaporator 27 and the pressure of the refrigerant in evaporator 27 to regulate the flow of refrigerant into evaporator 27 to keep the combination of temperature and pressure within some specified bounds. In a preferred embodiment, expansion valve 26 is an electronic expansion valve and sensor 31 is a temperature transducer such as a thermocouple or thermistor. In another embodiment, expansion valve 26 is a mechanical thermal expansion valve and sensor 31 includes a small tube that terminates in a pressure vessel filled with a refrigerant that differs from the refrigerant running through refrigeration system 20A. As refrigerant from evaporator 27 flows past sensor 31 on its way toward multi-stage compressor unit 10A, the pressure vessel will either heat up or cool down, thereby changing the pressure within the pressure vessel. As the pressure in the pressure vessel changes, sensor 31 sends a signal to expansion valve 26 to modify the pressure drop caused by the valve. Similarly, in the case of the electronic expansion valve, sensor 31 sends an electrical signal to expansion valve 26 which responds in a similar manner to regulate refrigerant flow. For example, if a return gas coming from evaporator 27 is too hot, sensor 31 will then heat up and send a signal to expansion valve 26, causing the valve to open further and allow more refrigerant per unit time to flow through evaporator 27, thereby reducing the heat of the refrigerant exiting evaporator 27.

Economizer circuit 25A also includes sensor 31A that operates in a similar manner to sensor 31. However, sensor 31A senses temperature along economizer return path 46A and acts with expansion valve 30A to control the pressure drop within expansion valve 30A instead. It should also be noted that sensors other than the ones previously described may be substituted for sensors 31 and 31A.

By controlling the expansion valves 26 and 30A, the operation of refrigeration system 20A can be adjusted to meet the cooling demands and achieve optimum efficiency. In addition to adjusting the pressure drops associated with expansion valves 26 and 30A, the displacements of cylinders 36A, 36B, and 36C may also be adjusted to help achieve optimum efficiency of refrigeration system 20A.

FIG. 1B illustrates a graph relating enthalpy to pressure for the refrigeration system 20A of FIG. 1A. Vapor dome V is formed by a saturated liquid line and a saturated vapor line, and defines the state of the refrigerant at various points along the refrigeration cycle. Underneath vapor dome V, all states involve both liquid and vapor coexisting at the same time. At the very top of vapor dome V is the critical point. The critical point is defined by the highest pressure where saturated liquid and saturated vapor coexist. In general, compressed liquids are located to the left of vapor dome V, while superheated vapors are located to the right of vapor dome V.

Once again, in FIG. 1B, the main refrigerant path is the loop defined by the points 1, 2, 3, 4, and 5, and the first economized path is the loop defined by the points 4A, 5A, 6A, and 7A. The cycle begins in the main path at point 1, where the refrigerant is at a low pressure and high enthalpy prior to entering multi-stage compressor unit 10A. After a first stage of compression within cylinder 36A of two-stage compressor sub-unit 32, both the enthalpy and pressure increase as shown by point 2. After a second stage of compression within cylinder 36B, the refrigerant exits multi-stage compressor unit 10A at high pressure and even higher enthalpy, as shown by point 3. Then, as the refrigerant flows through heat rejecting heat exchanger 24, enthalpy decreases while pressure remains constant. Prior to entering first economizer heat exchanger 28A, the refrigerant splits into a main portion and a first economized portion as shown by point 4A. The main portion is then throttled in main expansion valve 26, decreasing pressure as shown by point 5. Finally, the main portion of the refrigerant is evaporated, exiting evaporator 27 at a higher enthalpy as shown by point 1.

As stated previously, the first economized portion splits off of the main portion as indicated by point 4A. The first economized portion is throttled to a lower pressure in expansion valve 30A as shown by point 5A. The first economized portion of the refrigerant then exchanges heat with the main portion in first economizer heat exchanger 28A, cooling down the main portion of the refrigerant as indicated by point 4, and heating up the first economized portion of the refrigerant as indicated by point 6A. The first economized portion is then compressed within single-stage compressor sub-unit 34 and merged with the main portion of the refrigerant discharged from two-stage compressor sub-unit 32, as shown by point 7A.

As shown in FIG. 1B, cylinders 36A, 36B, and 36C of multi-stage compressor unit 10A are configured to receive and compress refrigerant to different pressures. In particular, cylinder 36A receives and compresses refrigerant from the main refrigerant path to an intermediate pressure, as indicated by point 2. Then, cylinder 36B receives and compresses the refrigerant from an intermediate pressure to an exit pressure, as indicated by point 3. Similarly, cylinder 36C receives and compresses refrigerant from the first economized refrigerant path to an exit pressure, as indicated by point 7A. As shown in FIG. 1B, the exit pressure of cylinder 36C is substantially equivalent to the exit pressure of cylinder 36B. In refrigeration system 20A, the exit pressures are determined by the inlet pressure required by heat rejecting heat exchanger 24.

FIG. 2A illustrates a schematic diagram of multi-stage compressor unit 10B connected to refrigeration system 20B. Multi-stage compressor unit 10B is similar to multi-stage compressor unit 10A. However, as will be discussed in more detail below, two-stage compressor sub-unit 32 further includes interstage port 48 configured to receive refrigerant from an economizer circuit to cool down the refrigerant in the main refrigerant path prior to a second stage of compression. Refrigeration system 20B is similar to refrigeration system 20A, but further includes second economizer circuit 25B. Second economizer circuit 25B includes second economizer heat exchanger 28B, expansion valve 30B, and sensor 31B.

In refrigeration system 20B, three distinct refrigerant paths are formed by connection of the various elements in the system. A main refrigerant path is created by a loop defined by the points 1, 2, 3, 4, 5, and 6. A first economized refrigerant path is created by a loop defined by the points 5A, 6A, 7A, 3, and 4. Finally, a second economized refrigerant path is created by a loop defined by the points 5B, 6B, 7B, and 8B.

In reference to the main refrigerant path, after refrigerant exits two-stage compressor sub-unit 32 at high pressure and enthalpy through discharge port 39 (point 4), the refrigerant loses heat in heat rejecting heat exchanger 24, exiting heat rejecting heat exchanger 24 at low enthalpy and high pressure (point 5A). The refrigerant then splits into two flow paths 40A and 42A prior to entering first economizer heat exchanger 28A. The main path continues along paths 40A and 40B through first economizer heat exchanger 28A (point 5B) and second economizer heat exchanger 28B (point 5), respectively. As the refrigerant in path 40A flows through first economizer heat exchanger 28A, it is cooled by the refrigerant in path 42A of the first economized path. Similarly, as the refrigerant in path 40B flows through second economizer heat exchanger 28B, it is cooled by the refrigerant in path 42B of the second economized path.

Refrigerant from path 40B is then throttled in main expansion valve 26. After going through an expansion process within main expansion valve 26 (point 6), the refrigerant is a two-phase liquid-vapor mixture and is directed toward evaporator 27. After evaporation of the remainder of the liquid (point 1), the refrigerant enters two-stage compressor sub-unit 32 through suction port 37. The refrigerant is compressed within cylinder 36A, which is the first stage of two-stage compressor sub-unit 32, and is then directed out of discharge port 50 (point 2), where it merges with the cooler refrigerant from economizer return path 46A that is injected into interstage port 48 (point 3). Thus, the refrigerant from economizer return path 46A functions to cool down the refrigerant discharged from cylinder 36A prior to the second stage of compression within cylinder 36B. After the second stage of compression, the refrigerant is discharged through discharge port 39 (point 4).

In reference to the first economized path, after refrigerant exits heat rejecting heat exchanger 24 at low enthalpy and high pressure (point 5A) and splits into two flow paths 40A and 42A, the first economized path continues along path 42A. In path 42A, the refrigerant is throttled to a lower pressure by economizer expansion valve 30A (point 6A) prior to flowing through first economizer heat exchanger 28A. The refrigerant from path 42A that flowed through first economizer heat exchanger 28A (point 7A) is then directed along economizer return path 46A and injected into interstage port 48 of two-stage compressor sub-unit 32 where it merges with refrigerant flowing through the main path to cool down the refrigerant (point 3) prior to a second stage of compression in cylinder 36B.

In reference to the second economized path, after being cooled in the higher pressure first economizer heat exchanger 28A (point 5B), the refrigerant in path 40A splits into two flow paths 40B and 42B. The second economized path continues along flow path 42B where the refrigerant is throttled to a lower pressure by economizer expansion valve 30B (point 6B) prior to flowing through second economizer heat exchanger 28B. The refrigerant from path 42B that flowed through second economizer heat exchanger 28B (point 7B) is then directed along economizer return path 46B and injected into suction port 52 of single-stage compressor sub-unit 34 for compression in single-stage compressor sub-unit 34. After compression within single-stage compressor sub-unit 34, refrigerant is discharged through discharge port 54 (point 8B) where it merges with the refrigerant discharged from two-stage compressor sub-unit 32.

FIG. 2B illustrates a graph relating enthalpy to pressure for the refrigeration system 20B of FIG. 2A. As shown in FIG. 2B, the main refrigerant path is the loop defined by the points 1, 2, 3, 4, 5, and 6; the first economized path is the loop defined by the points 5A, 6A, 7A, 3, and 4; and the second economized path is the loop defined by the points 5B, 6B, 7B, and 8B.

As shown in FIG. 2B, cylinders 36A, 36B, and 36C of multi-stage compressor unit 10B are configured to receive and compress refrigerant to different pressures. In particular, cylinder 36A receives and compresses refrigerant from the main refrigerant path to an intermediate pressure, as indicated by point 2. Then, cylinder 36B receives and compresses refrigerant from the main refrigerant path and the first economized path from an intermediate pressure to an exit pressure, as indicated by point 4. Similarly, cylinder 36C receives and compresses refrigerant from the second economized refrigerant path to an exit pressure, as indicated by point 8B. As shown in FIG. 2B, the exit pressure of cylinder 36C is substantially equivalent to the exit pressure of cylinder 36B.

FIG. 3A illustrates a schematic diagram of multi-stage compressor unit 10C connected to refrigeration system 20C. Multi-stage compressor unit 10C is similar to multi-stage compressor unit 10B. However, as will be discussed in more detail below, single-stage compressor sub-unit 34 is configured to discharge into first economizer return path 46A instead of into heat rejecting heat exchanger 24, as depicted by multi-stage compressor unit 10B of FIG. 2A.

In refrigeration system 20C, three distinct refrigerant paths are formed by connection of the various elements in the system. A main refrigerant path is created by a loop defined by the points 1, 2, 3, 4, 5, and 6. A first economized refrigerant path is created by a loop defined by the points 5A, 6A, 7A, 3, and 4. Finally, a second economized refrigerant path is created by a loop defined by the points 5B, 6B, 7B, 8B, 3, and 4.

In reference to the main refrigerant path, after refrigerant exits two-stage compressor sub-unit 32 at high pressure and enthalpy through discharge port 39 (point 4), the refrigerant loses heat in heat rejecting heat exchanger 24, exiting heat rejecting heat exchanger 24 at low enthalpy and high pressure (point 5A). The refrigerant then splits into two flow paths 40A and 42A prior to entering first economizer heat exchanger 28A. The main path continues along paths 40A and 40B through first economizer heat exchanger 28A (point 5B) and second economizer heat exchanger 28B (point 5), respectively. As the refrigerant in path 40A flows through first economizer heat exchanger 28A, it is cooled by the refrigerant in path 42A of the first economized path. Similarly, as the refrigerant in path 40B flows through second economizer heat exchanger 28B, it is cooled by the refrigerant in path 42B of the second economized path.

Refrigerant from path 40B is then throttled in main expansion valve 26. After going through an expansion process within main expansion valve 26 (point 6), the refrigerant is a two-phase liquid-vapor mixture and is directed toward evaporator 27. After evaporation of the remainder of the liquid (point 1), the refrigerant enters two-stage compressor sub-unit 32 through suction port 37. The refrigerant is compressed within cylinder 36A, which is the first stage of two-stage compressor sub-unit 32, and is then directed out discharge port 50 (point 2), where it merges with the cooler refrigerant from economizer return path 46A that is injected into interstage port 48 (point 3). Thus, the refrigerant from economizer return path 46A functions to cool down the refrigerant discharged from cylinder 36A prior to the second stage of compression within cylinder 36B. After the second stage of compression, the refrigerant is discharged through discharge port 39 (point 4).

In reference to the first economized path, after refrigerant exits heat rejecting heat exchanger 24 at low enthalpy and high pressure (point 5A) and splits into two flow paths 40A and 42A, the first economized path continues along path 42A. In path 42A, the refrigerant is throttled to a lower pressure by economizer expansion valve 30A (point 6A) prior to flowing through first economizer heat exchanger 28A. The refrigerant from path 42A that flowed through first economizer heat exchanger 28A (point 7A) is then directed along economizer return path 46A and injected into interstage port 48 of two-stage compressor sub-unit 32 where it merges with refrigerant flowing through the main path to cool down the refrigerant (point 3) prior to a second stage of compression in cylinder 36B.

In reference to the second economized path, after being cooled in the higher pressure first economizer heat exchanger 28A (point 5B), the refrigerant in path 40A splits into two flow paths 40B and 42B. The second economized path continues along flow path 42B where the refrigerant is throttled to a lower pressure by economizer expansion valve 30B (point 6B) prior to flowing through second economizer heat exchanger 28B. The refrigerant from path 42B that flowed through second economizer heat exchanger 28B (point 7B) is then directed along economizer return path 46B and injected into suction port 52 of single-stage compressor sub-unit 34 for compression in single-stage compressor sub-unit 34. After compression within single-stage compressor sub-unit 34, the refrigerant is discharged through discharge port 54 where it is mixed with the refrigerant in economizer return path 46A (point 8B) prior to injection into interstage port 48 of two-stage compressor sub-unit 32 (point 3).

FIG. 3B illustrates a graph relating enthalpy to pressure for the refrigeration system 20C of FIG. 3A. As shown in FIG. 3B, the main refrigerant path is the loop defined by the points 1, 2, 3, 4, 5, and 6; the first economized path is the loop defined by the points 5A, 6A, 7A, 3, and 4; and the second economized path is the loop defined by the points 5B, 6B, 7B, 8B, 3, and 4.

As shown in FIG. 3B, cylinders 36A, 36B, and 36C of multi-stage compressor unit 10C are configured to receive and compress refrigerant to different pressures. In particular, cylinder 36A receives and compresses refrigerant from the main refrigerant path to an intermediate pressure, as indicated by point 2. Similarly, cylinder 36C receives and compresses refrigerant from the second economized refrigerant path to an exit pressure, as indicated by point 8B. Then, cylinder 36B receives and compresses refrigerant from the main refrigerant path, the first economized path, and the second economized path to an exit pressure, as indicated by point 4. As shown in FIG. 3B, the exit pressure of cylinder 36C is substantially equivalent to the intermediate pressure of cylinder 36A.

FIG. 4A illustrates a schematic diagram of multi-stage compressor unit 10D connected to refrigeration system 20D. Multi-stage compressor unit 10D is similar to multi-stage compressor unit 10A. However, multi-stage compressor unit 10D further includes single-stage compressor sub-unit 35 having cylinder 36D.

Refrigeration system 20D is similar to refrigeration system 20C, except that interstage port 48 is replaced by intercooler 49, which is configured to cool the main portion of the refrigerant between the first and second stages of compression in two-stage compressor sub-unit 32.

In refrigeration system 20D, three distinct refrigerant paths are formed by connection of the various elements in the system. A main refrigerant path is created by a loop defined by the points 1, 2, 3, 4, 5, and 6. A first economized refrigerant path is created by a loop defined by the points 5A, 6A, 7A, and 8A. Finally, a second economized refrigerant path is created by a loop defined by the points 5B, 6B, 7B, and 8B.

In reference to the main refrigerant path, after refrigerant exits two-stage compressor sub-unit 32 at high pressure and enthalpy through discharge port 39 (point 4), the refrigerant loses heat in heat rejecting heat exchanger 24, exiting heat rejecting heat exchanger 24 at low enthalpy and high pressure (point 5A). The refrigerant then splits into two flow paths 40A and 42A prior to entering first economizer heat exchanger 28A. The main path continues along paths 40A and 40B through first economizer heat exchanger 28A (point 5B) and second economizer heat exchanger 28B (point 5), respectively. As the refrigerant in path 40A flows through first economizer heat exchanger 28A, it is cooled by the refrigerant in path 42A of the first economized path. Similarly, as the refrigerant in path 40B flows through second economizer heat exchanger 28B, it is cooled by the refrigerant in path 42B of the second economized path.

Refrigerant from path 40B is then throttled in main expansion valve 26. After going through an expansion process within main expansion valve 26 (point 6), the refrigerant is a two-phase liquid-vapor mixture and is directed toward evaporator 27. After evaporation of the remainder of the liquid (point 1), the refrigerant enters two-stage compressor sub-unit 32 through suction port 37. The refrigerant is compressed within cylinder 36A, which is the first stage of two-stage compressor sub-unit 32, and is then directed out discharge port 50 (point 2), where it flows through intercooler 49 prior to a second stage of compression in cylinder 36B. Intercooler 49 is configured to cool down the refrigerant discharged from cylinder 36A prior to the second stage of compression within cylinder 36B. After the second stage of compression, the refrigerant is discharged through discharge port 39 (point 4).

In reference to the first economized path, after refrigerant exits heat rejecting heat exchanger 24 at low enthalpy and high pressure (point 5A) and splits into two flow paths 40A and 42A, the first economized path continues along path 42A. In path 42A, the refrigerant is throttled to a lower pressure by economizer expansion valve 30A (point 6A) prior to flowing through first economizer heat exchanger 28A. The refrigerant from path 42A that flowed through first economizer heat exchanger 28A (point 7A) is then directed along economizer return path 46A and injected into suction port 52 of single-stage compressor sub-unit 34 for compression in single-stage compressor sub-unit 34. After compression within single-stage compressor sub-unit 34, the refrigerant is discharged through discharge port 54 (point 8A) where it merges with the refrigerant discharged from two-stage compressor sub-unit 32 and single-stage compressor sub-unit 35.

In reference to the second economized path, after being cooled in the higher pressure first economizer heat exchanger 28A (point 5B), the refrigerant in path 40A splits into two flow paths 40B and 42B. The second economized path continues along flow path 42B where the refrigerant is throttled to a lower pressure by economizer expansion valve 30B (point 6B) prior to flowing through second economizer heat exchanger 28B. The refrigerant from path 42B that flowed through second economizer heat exchanger 28B (point 7B) is then directed along economizer return path 46B and injected into suction port 56 of single-stage compressor sub-unit 35 for compression in single-stage compressor sub-unit 35. After compression within single-stage compressor sub-unit 35, the refrigerant is discharged through discharge port 58 (point 8B) where it merges with the refrigerant discharged from two-stage compressor sub-unit 32 and single-stage compressor sub-unit 34.

FIG. 4B illustrates a graph relating enthalpy to pressure for the refrigeration system 20D of FIG. 4A. As shown in FIG. 4B, the main refrigerant path is the loop defined by the points 1, 2, 3, 4, 5, and 6; the first economized path is the loop defined by the points 5A, 6A, 7A, and 8A; and the second economized path is the loop defined by the points 5B, 6B, 7B, and 8B.

As shown in FIG. 4B, cylinders 36A, 36B, 36C, and 36D of multi-stage compressor unit 10D are configured to receive and compress refrigerant to different pressures. In particular, cylinder 36A receives and compresses refrigerant from the main refrigerant path to an intermediate pressure, as indicated by point 2. After being cooled within intercooler 49, cylinder 36B receives and compresses refrigerant from the main refrigerant path from an intermediate pressure to an exit pressure, as indicated by point 4. Cylinder 36C receives and compresses refrigerant from the first economized refrigerant path to an exit pressure, as indicated by point 8A. Similarly, cylinder 36D receives and compresses refrigerant from the second economized refrigerant path to an exit pressure, as indicated by point 8B. As shown in FIG. 4B, the exit pressures of cylinders 36C and 36D are substantially equivalent to the exit pressure of cylinder 36B.

FIG. 5A illustrates a schematic diagram of multi-stage compressor unit 10E connected to refrigeration system 20E. In addition to two-stage compressor sub-unit 32, multi-stage compressor unit 10E further includes two-stage compressor sub-unit 70. Two-stage compressor sub-unit 70 includes cylinders 36E and 36F connected in series. Refrigeration system 20E is similar to refrigeration system 20D, except that third economizer circuit 25C is added to the system.

In refrigeration system 20E, four distinct refrigerant paths are formed by connection of the various elements in the system. A main refrigerant path is created by a loop defined by the points 1, 2, 3, 4, 5, and 6. A first economized refrigerant path is created by a loop defined by the points 5A, 6A, 7A, 3, and 4. A second economized refrigerant path is created by a loop defined by the points 5B, 6B, 7B, 9, and 10. Finally, a third economized refrigerant path is created by a loop defined by the points 5C, 6C, 7C, 8C, 9, and 10.

In reference to the main refrigerant path, after refrigerant exits two-stage compressor sub-unit 32 at high pressure and enthalpy through discharge port 39 (point 4), the refrigerant loses heat in heat rejecting heat exchanger 24, exiting heat rejecting heat exchanger 24 at low enthalpy and high pressure (point 5A). The refrigerant then splits into two flow paths 40A and 42A prior to entering first economizer heat exchanger 28A. The main path continues along paths 40A, 40B, and 40C through first economizer heat exchanger 28A (point 5B), second economizer heat exchanger 28B (point 5C), and third economizer heat exchanger 28C (point 5), respectively. As the refrigerant in path 40A flows through first economizer heat exchanger 28A, it is cooled by the refrigerant in path 42A of the first economized path. As the refrigerant in path 40B flows through second economizer heat exchanger 28B, it is cooled by the refrigerant in path 42B of the second economized path. Finally, as the refrigerant in path 40C flows through third economizer heat exchanger 28C, it is cooled by the refrigerant in path 42C of the third economized path.

Refrigerant from path 40C is then throttled in main expansion valve 26. After going through an expansion process within main expansion valve 26 (point 6), the refrigerant is a two-phase liquid-vapor mixture and is directed toward evaporator 27. After evaporation of the remainder of the liquid (point 1), the refrigerant enters two-stage compressor sub-unit 32 through suction port 37. The refrigerant is compressed within cylinder 36A, which is the first stage of two-stage compressor sub-unit 32, and is then directed out discharge port 50 (point 2), where it merges with the cooler refrigerant from economizer return path 46A that is injected into interstage port 48 (point 3). Thus, the refrigerant from economizer return path 46A functions to cool down the refrigerant discharged from cylinder 36A prior to the second stage of compression within cylinder 36B. After the second stage of compression, the refrigerant is discharged through discharge port 39 (point 4).

In reference to the first economized path, after refrigerant exits heat rejecting heat exchanger 24 at low enthalpy and high pressure (point 5A) and splits into two flow paths 40A and 42A, the first economized path continues along path 42A. In path 42A, the refrigerant is throttled to a lower pressure by economizer expansion valve 30A (point 6A) prior to flowing through first economizer heat exchanger 28A. The refrigerant from path 42A that flowed through first economizer heat exchanger 28A (point 7A) is then directed along economizer return path 46A and injected into interstage port 48 of two-stage compressor sub-unit 32 where it merges with refrigerant flowing through the main path to cool down the refrigerant (point 3) prior to a second stage of compression in cylinder 36B.

In reference to the second economized path, after being cooled in the higher pressure first economizer heat exchanger 28A, the refrigerant in path 40A splits into two flow paths 40B and 42B (point 5B). The second economized path continues along flow path 42B where the refrigerant is throttled to a lower pressure by economizer expansion valve 30B prior to flowing through second economizer heat exchanger 28B (point 6B). The refrigerant from path 42B that flowed through second economizer heat exchanger 28B (point 7B) is then directed along economizer return path 46B and injected into interstage port 72 of two-stage compressor sub-unit 70 where it mixes with refrigerant exiting discharge port 74 (point 9) to cool down the refrigerant prior to a second stage of compression in cylinder 36F.

In reference to the third economized path, after being cooled in the higher pressure second economizer heat exchanger 28B, the refrigerant in path 40B splits into two flow paths 40C and 42C (point 5C). The third economized path continues along flow path 42C where the refrigerant is throttled to a lower pressure by economizer expansion valve 30C prior to flowing through third economizer heat exchanger 28C (point 6C). The refrigerant from path 42C that flowed through third economizer heat exchanger 28C (point 7C) is then directed along economizer return path 46C and injected into suction port 76 of two-stage compressor sub-unit 70. After a first stage of compression in cylinder 36E (point 8C), the refrigerant is cooled prior to a second stage of compression by the refrigerant from economizer return path 46B that was injected into interstage port 72 (point 9). After the second stage of compression in cylinder 36F, the refrigerant is discharged through discharge port 78 (point 10), where it merges with the compressed refrigerant discharged from two-stage compressor sub-unit 32.

FIG. 5B illustrates a graph relating enthalpy to pressure for the refrigeration system 20E of FIG. 5A. As shown in FIG. 5B, the main refrigerant path is the loop defined by the points 1, 2, 3, 4, 5, and 6; the first economized path is the loop defined by the points 5A, 6A, 7A, 3, and 4; the second economized path is the loop defined by the points 5B, 6B, 7B, 9, and 10; and the third economized path is the loop defined by the points 5C, 6C, 7C, 8C, 9, and 10.

As shown in FIG. 5B, cylinders 36A, 36B, 36E, and 36F of multi-stage compressor unit 10E are configured to receive and compress refrigerant to different pressures. In particular, cylinder 36A receives and compresses refrigerant from the main refrigerant path to an intermediate pressure, as indicated by point 2. Then, cylinder 36B receives and compresses refrigerant from the main refrigerant path and the first economized path from the intermediate pressure to an exit pressure, as indicated by point 4. Similarly, cylinder 36E receives and compresses refrigerant from the third economized refrigerant path to an intermediate pressure, as indicated by point 8C. Cylinder 36F then receives and compresses refrigerant from the second and third economized paths from the intermediate pressure to an exit pressure, as indicated by point 10. As shown in FIG. 5B, the exit pressure of cylinder 36B is substantially equivalent to the exit pressure of cylinder 36F.

Although each of the embodiments of a multi-stage compressor unit shown and described above have been connected to a refrigeration system that includes one or more economizer circuits, the multi-stage compressor unit of the present invention may also be used in refrigerating systems that do not include economizer circuits. FIG. 6A illustrates a schematic diagram of multi-stage compressor unit 10F connected to refrigeration system 20F, which includes heat rejecting heat exchanger 24, first expansion valve 26, first evaporator 27, first sensor 31, second expansion valve 126, second evaporator 127, and second sensor 131. Multi-stage compressor unit 10F includes two-stage compressor sub-unit 32 and single-stage compressor sub-unit 34. Two-stage compressor sub-unit 32 includes cylinders 36A and 36B connected in series, while single-stage compressor sub-unit 34 includes cylinder 36C.

In refrigeration system 20F, two distinct refrigerant paths are formed by connection of the various elements in the system. A first main refrigerant path is created by a loop defined by the points 1, 2, 3, 4, and 5. A second main refrigerant path is created by a loop defined by the points 4, 5A, 6A, and 7A.

In reference to the first main refrigerant path, after refrigerant exits two-stage compressor sub-unit 32 at high pressure and enthalpy through discharge port 39 (point 3), the refrigerant loses heat in heat rejecting heat exchanger 24, exiting heat rejecting heat exchanger 24 at low enthalpy and high pressure (point 4). The refrigerant in the main path is then throttled in first expansion valve 26. After going through an expansion process within first expansion valve 26 (point 5), the refrigerant is a two-phase liquid-vapor mixture and is directed toward first evaporator 27. After evaporation of the remainder of the liquid (point 1), the refrigerant enters two-stage compressor sub-unit 32 through suction port 37. The refrigerant is compressed within cylinder 36A, which is the first stage of two-stage compressor sub-unit 32, and is then directed out discharge port 50 (point 2). After the second stage of compression in cylinder 36B, the refrigerant is discharged through discharge port 39 (point 3).

In reference to the second main refrigerant path, after exiting heat rejecting heat exchanger 24, the refrigerant is throttled in second expansion valve 126. After going through an expansion process within second expansion valve 126 (point 5A), the refrigerant is a two-phase liquid-vapor mixture and is directed toward second evaporator 127. After evaporation in second evaporator 127 (point 6A), the refrigerant enters single-stage compressor sub-unit 34 through suction port 52. The refrigerant is compressed within cylinder 36C, and is then directed out discharge port 54 point 7A) where it mixes with the refrigerant discharged through discharge port 39 of two-stage compressor sub-unit 32.

FIG. 6B illustrates a graph relating enthalpy to pressure for the refrigeration system 20F of FIG. 6A. As shown in FIG. 6B, the first main refrigerant path is the loop defined by the points 1, 2, 3, 4, and 5, and the second main refrigerant path is the loop defined by the points 4, 5A, 6A, and 7A.

As shown in FIG. 6B, cylinders 36A, 36B, and 36C of multi-stage compressor unit 10F are configured to receive and compress refrigerant to different pressures. In particular, cylinder 36A receives and compresses refrigerant from the first main refrigerant path to an intermediate pressure, as indicated by point 2. Then, cylinder 36B receives and compresses the refrigerant from the intermediate pressure to an exit pressure, as indicated by point 3. Similarly, cylinder 36C receives and compresses refrigerant from the second main refrigerant path to an exit pressure, as indicated by point 7A. As shown in FIG. 6B, the exit pressure of cylinder 36C is substantially equivalent to the exit pressure of cylinder 36B.

While the alternative embodiments of the multi-stage compressor unit have been described as including a number of compressor sub-units ranging from two to three, it should be understood that a multi-stage compressor unit with more than three compressor sub-units is within the intended scope of the present invention. Furthermore, although the embodiments of the multi-stage compressor unit were described as including only single-stage and two-stage compressor sub-units, compressor sub-units having more than two stages are within the intended scope of the present invention. Thus, single-stage and two-stage compressor sub-units were shown merely for purposes of example and not for limitation. In addition, alternative embodiments that include compressor sub-units connected in various combinations other than those depicted above are also contemplated.

Although the multi-stage compressor unit of the present invention is useful to increase system efficiency in a refrigeration system using any type of refrigerant, it is especially useful in refrigeration systems that utilize transcritical refrigerants, such as carbon dioxide. Because carbon dioxide is such a low critical temperature refrigerant, refrigeration systems using carbon dioxide typically run transcritical. Furthermore, because carbon dioxide is such a high pressure refrigerant, there is more opportunity to provide multiple pressure steps between the high and low pressure portions of the circuit to include multiple economizers and multiple compressor cylinders, each of which contributes to increase the efficiency of the system. Thus, the multi-stage compressor unit of the present invention may be used to increase the efficiency of systems utilizing transcritical refrigerants such as carbon dioxide, making their efficiency comparable to that of typical refrigerants. However, the multi-stage compressor unit of the present invention is useful to increase the efficiency in refrigeration systems using any refrigerant, including those that run subcritical as well as those that run transcritical.

Although the present invention has been described with reference to preferred embodiments, workers skilled in the art will recognize that changes may be made in form and detail without departing from the spirit and scope of the invention. 

1. A multi-stage compressor unit for a refrigeration system configured to circulate a refrigerant, the multi-stage compressor unit comprising: a first compressor sub-unit having a first stage and a second stage, wherein the first and second stages of the first compressor sub-unit each have a suction port and a discharge port, and wherein the first compressor sub-unit is configured to receive and compress a first portion of the refrigerant from an evaporator; and a second compressor sub-unit in parallel with the first compressor sub-unit and having a first stage, wherein the first stage of the second compressor sub-unit has a suction port and a discharge port, and wherein the second compressor sub-unit is configured to compress a second portion of the refrigerant prior to the compressed second portion being combined with the compressed first portion.
 2. The multi-stage compressor unit of claim 1, wherein the first stage of the second compressor sub-unit is configured to receive the second portion of the refrigerant from a second evaporator.
 3. The multi-stage compressor unit of claim 1, wherein the first stage of the second compressor sub-unit is configured to receive the second portion of the refrigerant from a first economizer circuit.
 4. The multi-stage compressor unit of claim 3, wherein the first compressor sub-unit further comprises an interstage port disposed between the discharge port of the first stage and the suction port of the second stage, and wherein the interstage port is configured to receive a third portion of the refrigerant from a second economizer circuit.
 5. The multi-stage compressor unit of claim 1, wherein the first compressor sub-unit and the second compressor sub-unit comprise reciprocating compressors.
 6. The multi-stage compressor unit of claim 1, wherein the refrigerant is a transcritical refrigerant.
 7. The multi-stage compressor unit of claim 1, and further comprising an intercooler configured to cool the first portion of the refrigerant between the first and second stages of the first compressor sub-unit.
 8. The multi-stage compressor unit of claim 1, wherein the second compressor sub-unit is configured to discharge into an interstage port of the first compressor sub-unit.
 9. The multi-stage compressor unit of claim 1, wherein the second compressor sub-unit further comprises a second stage having a suction port and a discharge port, and wherein the first and second stages of the second compressor sub-unit are connected in series.
 10. A multi-stage compressor unit for a refrigeration system having a first economizer circuit and configured to circulate a refrigerant, the multi-stage compressor unit comprising: a first compressor sub-unit having a first stage and a second stage, wherein the first compressor sub-unit is configured to receive and compress a first portion of the refrigerant from an evaporator; and a second compressor sub-unit in parallel with the first compressor sub-unit, wherein the second compressor sub-unit is configured to compress a second portion of the refrigerant from the first economizer circuit prior to the compressed second portion being combined with the compressed first portion.
 11. The multi-stage compressor unit of claim 10, and further comprising an intercooler configured to cool the first portion of the refrigerant between the first and second stages of the first compressor sub-unit.
 12. The multi-stage compressor unit of claim 10, wherein the first compressor sub-unit is configured to discharge refrigerant at a first exit pressure, and wherein the second compressor sub-unit is configured to discharge refrigerant at a second exit pressure.
 13. The multi-stage compressor unit of claim 12, wherein the first and second exit pressures are substantially equivalent.
 14. The multi-stage compressor unit of claim 10, wherein the first compressor sub-unit further comprises an interstage port disposed between the first and second stages and configured to receive a third portion of the refrigerant from a second economizer circuit, and wherein the second stage of the first compressor sub-unit is configured to compress a mixture of the first and third portions of the refrigerant.
 15. The multi-stage compressor unit of claim 10, wherein the second compressor sub-unit is a single-stage compressor.
 16. The multi-stage compressor unit of claim 10, wherein the second compressor sub-unit is a two-stage compressor.
 17. The multi-stage compressor unit of claim 16, wherein the second compressor sub-unit further comprises an interstage port configured to receive a fourth portion of the refrigerant from a third economizer circuit.
 18. A multi-stage compressor unit for a refrigeration system configured to circulate a refrigerant, the multi-stage compressor unit comprising: a first compressor sub-unit having a first stage, a second stage, and an interstage port disposed between the first and second stages, wherein the first stage of the first compressor sub-unit is configured to compress a first portion of the refrigerant to an intermediate pressure, and wherein the second stage of the first compressor sub-unit is configured to compress the first portion of the refrigerant to an exit pressure of the first compressor sub-unit; and a second compressor sub-unit having a first stage, wherein the second compressor sub-unit is configured to compress a second portion of the refrigerant to an exit pressure of the second compressor sub-unit prior to the compressed second portion being combined with the compressed first portion.
 19. The multi-stage compressor unit of claim 18, wherein the exit pressures of the first compressor sub-unit and the second compressor sub-unit are substantially equivalent.
 20. The multi-stage compressor unit of claim 18, wherein the second compressor sub-unit discharges the second portion of the refrigerant into the interstage port of the first compressor sub-unit, and wherein the exit pressure of the second compressor sub-unit is substantially equivalent to the intermediate pressure of the first compressor sub-unit. 